The present invention relates to the field of seismic exploration. More particularly, the invention relates to a method for determining optimal explosive characteristics for specific seismic survey conditions.
Holes are drilled in rock for excavation blasting, mining operations, and many other purposes. For example, explorative searches for hydrocarbons, minerals, and other products require the physical penetration of geologic formations. Seismic operations typically detonate explosive charges to generate shock wave source signals for penetrating subsurface geologic formations. The shock waves are reflected from subsurface geologic structures and interfaces and the reflected energy is detected with sensors such as geophones at the surface. These transducers reduce the reflected energy into signals which are recorded for processing.
In many land-based geophysical seismic operations, vibrator trucks contact the soil and discharge energy into subsurface geologic formations. However, survey regions frequently comprise mountainous, tropical, or other regions inaccessible to seismic trucks. Because of accessibility constraints and the large source energy provided by explosive materials, explosive charges detonated in shot-holes provide a preferred source of seismic source energy. Shot holes up to four inches wide and between two and thirty meters deep are commonly drilled in surface geologic formations to allow placement of the explosives. The explosive charges are typically placed in the bottom of the shot-hole and are detonated to generate shock waves transmitted into the subsurface geologic formations.
Seismic shot-holes require different parameters than excavation blast holes because the objective of shot-holes is not to displace or fracture rock, but to efficiently transfer elastic shock wave energy downwardly into subsurface geologic formations. Accordingly, shot-hole equipment and drilling techniques are relatively specialized.
The diameter of conventional explosive charges is smaller than the shot-hole diameter to facilitate placement of the explosives into the lower shot-hole end. The resulting annulus between the explosive charge and the shot-hole wall often reduces the efficiency with which the shock wave energy is transmitted to the subsurface geologic formations. Because of this reduction in efficiency, one technique promotes the use of gaseous explosives to eliminate the void space between the explosive and the borehole wall. U.S. Pat. No. 3,752,256 to Mollere (1973) disclosed a method for positioning a combustion chamber within soil to generate seismic source energy. U.S. Pat. No. 3,976,161 to Carman (1976) disclosed an auger for inserting an explosive gas mixture into loose soil.
A large portion of the shock wave energy is discharged upwardly through the shot-hole because of the relatively low resistance provided by the open hole. To limit this energy loss, plugs are placed in the shot-hole as shown in U.S. Pat. No. 4,066,125 to Bassani (1978). U.S. Pat. No. 4,736,796 to Amall et al. (1988) disclosed other techniques for sealing shot-holes with cement, gravel, and bentonite.
Explosives have provided a seismic energy source since the inception of seismic exploration, however little effort has been committed to the performance of explosive materials. Obstacles to explosive evaluation include unavailability of information regarding the impact of certain explosive parameters, the lack of effective techniques for field testing such parameters, lack of techniques for evaluating field test data and the high cost of conducting the multi-variant experiments required to evaluate the explosives.
Various techniques have been developed to control the shape and directivity of seismic energy discharges. U.S. Pat. No. 3,908,789 to Itria (1975) disclosed a technique for controlling the explosive material length. Control over detonation of an explosive material was disclosed in U.S. Pat. No. 4,053,027 to Oswald (1977), wherein a first and second energy pulse was generated during the same seismic event. Numerous publications have addressed the mechanics of energy wave transmission through various soil conditions.
Regional seismic operations require multiple shothole locations for a seismic survey, and large surveys can require thousands of shotholes. The average cost for each shothole multiplied by the number of shotholes significantly determines the economic efficiency of the survey and the data sets obtainable from a survey design. Seismic exploration is expensive to conduct and adequate data quality is sometimes difficult to obtain in certain geologic conditions. Drilling depth and on-site personnel and equipment time are significant cost factors. Accordingly, a need exists for improved techniques for efficiently determining the source parameters for seismic shotholes in areas inaccessible by heavy equipment.
The present invention provides a method for selecting a seismic energy source for use in a selected seismic survey area. The method comprises the steps of assessing selected physical properties of soil within the seismic survey area, of testing reaction of the soil response to selected seismic energy source characteristics, of generating a test model of a selected seismic energy source initiation within the soil, of estimating the far-field seismic response model of said seismic energy source initiation from said near-surface test model, of conducting a seismic event within the selected seismic survey area to measure seismic data initiated by said seismic event, and of comparing said far-field seismic response model to the seismic data initiated by said seismic event